34 stingrays die at Calgary zoo... wondering if you could think of an explanation?

Question
Hi, im very saddened to hear the sudden death of 34 cownose stingrays at Calgary zoo while 9 are still alive. Since you've so successfully helped me with other diagnostics and saved many of my fish (Thanks a bunch!!!) i was wondering if you could think of a diagnostic for the stingrays. maybe not an exact one just an idea. The site is:

http://www.theglobeandmail.com/servlet/story/RTGAM.20080512.wcalgary_zoo0512/BNS...

They had black blotches before they died and swam erratically. The deaths were close together and came on fast. It cant be an infection but maybe a poison? DO you know of anything that could cause sudden and mass death in fish? I just don't want to be left with an unsatisfactory answer and wanted a second opinion... so i came to the best i know! Thanks for all you've helped me with and all my fish enjoy it too!

Will

Answer
Hi Will,
Wow, what an interesting story. That's quite a tragedy though.

Although I'm not a vet and am not an expert on rays. I do know that with my experience in fish and fish problems. That the ray's behavior does almost exactly match the behavior of a common aquarium fish reacting to some poison in the water. The gill irritation and the skin blotches found on the rays are very much give-aways to me also.

Even though the water tests turned out fine. There may be something not yet discovered. It could be anything, some person may have introduced a poisonous substance in the water that hasn't been tracked yet. Someone may not have washed their hands, or purposely put something in the water. Chemicals in the air may have drifted into the tank and poisoned the water.

If this were an at-home aquarium I'd advise the aquarist to immediately change more than half the tank water. I just wonder if the zoo has changed the rays' water at all to hope in some way or another dilute the substance making them sick. I don't know if its possible, but putting large amounts of Carbon in the ray's filtration system would help get rid of whatever toxin is in the water.

I cannot rule out the possibility of an infection or parasitic infection. There may be some illness that has developed and has the severity enough to cause such alarmingly quick deaths.

I hope the Calgary Zoo find the real reason behind all the deaths and knows the way to correct it.

Thanks for the story and for all your wonderful comments! ;-)
Karen~